Electric arc torches



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ELECTRIC ARC TORCHES Filed Aug. 6, 19

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BY W0. l vzw United States Patent 3,223,822 ELECTRIC ARC TORCHES James A. Browning, Hanover, and Chapin A. Pratt, Plainfield, NIL, assignors to Thermal Dynamics Corporation, a corporation of New Hampshire Filed Aug. 6, 1963, Ser. No. 300,707 2 Claims. (Cl. 219-121) Our invention relates to electric arc torches, and particularly to those devices designed to produce plasma streams at extremely elevated temperatures.

In such devices, it is well known to establish an are between two electrodes, one of which may be a work piece, the arc usually passing through or part way down a nozzle passage. It is important to stabilize the arc, and this may be accomplished by a flow of gas along the arc path. A variety of gases may be employed, singly or in various mixtures depending on the particular application. These gases are heated, by their proximity to the arc, to high temperatures, at least some of the gas assuming the plasma state. The gas most removed from the arc column itself remains relatively cool, and forms a stabilizing sheath to define the arc in the desired configuration.

When operating plasma are devices at high arc powers demanded by modern technology, it is necessary to maintain a high voltage across the arc column between the cathode and the element acting as the anode. To obtain such high voltages it is necessary to use relatively high gas flows to stabilize and position the are through the required arc length. That is, are column length (and thus voltage) is a direct function of gas mass flow.

It will be appreciated then that the criteria leading to high are power and to maximizing gas heating are in conflict. This invention relates to a process and apparatus whereby these heretofore incompatible ends are both realized to a measure not before possible.

A complete understanding of our invention may be had from the following detailed explanation, read with reference to the accompanying drawings in which FIGURE 1 is a longitudinal view, in cross section, of a plasma torch utilizing the principles of our invention;

FIGURE 2 is a cross-sectional view taken on the lines 22 of FIGURE 1;

FIGURE 3 is a longitudinal view, in cross section, of an alternative structure for carrying out the invention; and

FIGURE 4 is a cross-sectional view taken on the lines 4--4 of FIGURE 3.

Referring now more particularly to FIGURE 1, an arc torch is shown in which electric power is delivered to an electrode 12 through lead 11 from a suitable power supply (not shown). An are column 19 is formed between electrode 12 and a nozzle electrode 13, which acts as the anode as shown. The electron circuit is completed through a lead 24 to the same power source to which the lead 11 is attached. The are column must be stabilized and, accordingly a plasma forming gas is introduced into annular cavity 16 through passage in the electrically nonconducting manifold 16. A ceramic swirl ring 18, having a plurality of tangential slots 17 therein, serves to introduce the gas into the nozzle passage 20 in a vortex flow. This stabilizing technique is explained in detail in US. Patent No. 3,027,446, issued on March 27, 1962, to James A. Browning, and no detailed description is here required. Sufiice to say, a relatively large gas flow is required to produce the long are column shown in FIGURE 1 and termination on the inside wall of the nozzle passage 20.

Much of this gas does not reach the plasma state, or even become ionized to any substantial degree. Heretofore, after serving its purpose as a stabilizer, the cool outer sheath gas emanated from the nozzle opening at 23 and there is served only to reduce the average temperature of the torch efiiux. In accordance with our invention we provide a series of apertures 25 tangentially disposed as shown in FIGURE 2, to bleed off this cooler peripheral gas at some point near the end of arc column 19. We also find it convenient and desirable to provide an annular cavity 28 in manifold 29 to collect the gases thus bled from the nozzle passage. The gas bleed rate may be regulated by a valve 26 in outlet pipe 27. With a positive pressure differential between the passage 20 and the atmosphere, and with the apertures 25 disposed in the same rotational sense as the vortex flow, an adequate bleed rate may usually be obtained. In some instances, suction apparatus may be attached to the outlet pipe 27 to achieve desired bleed rates.

The torch of FIGURE 1 may be operated in the trans ferred mode also. In such case, the anode function is served by a work piece as shown, and a lead 22 then is used for completion of the electrical circuit instead of the lead 24. In such case, of course, the arc column 19 continues along the dotted line to the work piece. In the transferred mode typical applications include welding, cutting or scarfing. In the non-transferred mode the torch may be employed to create a supersonic jet stream.

Under proper conditions, with a vortex gas flow, the arc column passes along the low-pressure vortex core. Only that small portion of the total gas flow in close proximity to the arc column itself is directly heated by the arc. The remaining whirling gas remains relatively cool and composes what may be described as a dark sheath zone surrounding the arc column. The peripheral gases receive heat by some convection and radiation from the hot core region. However, when all the gas found necessary for arc stabilization becomes a part of the efiluent, the total average efiluent temperature is substantially below that of the central hot gas stream. By bleeding the sheath gases off, the remaining flow, which becomes the efiiuent, is at significantly higher temperatures.

Our invention may be combined with a multiple downstream injection. This combination is particularly desirable, and often necessary, where are columns become very long. One embodiment of this system is shown in FIG- URE 3. Here we have a torch much like that previously discussed, but provided with a gas inlet manifold 32 having a cavity 33 and a series of tangential holes 34 open ing into the nozzle passage. A gas outlet manifold 35 is also provided with its cavity 36 and a corresponding series of outlet holes 37 leading to an outlet conduit 38. The inlet holes 34 and the outlet holes 37 will be seen to be alternately arranged along the arc column. Thus, after passing along a certain length of the nozzle 30, a portion of the sheath gas is bled off. This sheath gas has lost some of its whirling intensity and may be somewhat heated. It has therefore become less effective as a stabilizing mechanism for the arc column. Cool gas is again injected at the next inlet hole, downstream, and so on to a point near the end of the nozzle. Where the gas used is rather expensive (such as nitrogen or some of the rare gases) the bleed gas may be collected, cooled, com pressed, and re-introduced into the torch through the inlet conduit 31.

In addition to achieving higher average efilux tempera tures, our invention is particularly useful where it is desirable to reduce the total momentum of a hot gas jet prior to its impingement against a work surface, as the work piece in FIGURE 1. By bleeding off the cooler sheath as described, the proportion of low density hot gases is greatly increased. The cooler gases, while necessary for are stabilization, thus do not contribute the unwanted momentum to the jet stream. In welding operations, for example, this principle is often very important.

While the introduction of gases in a vortex has been shown, our invention is equally applicable to straight flows as shown, for example, in US. Patent No. 2,960,- 594. In such a flow mode, downstream bleed-off apertures are provided as before and the sheath gas, under pressure, thus escapes before reaching the nozzle opening but after it has performed its stabilizing function. In some cases suction apparatus may likewise be provided to assist in controlling the bleed-off rate and achieving the desired operating parameters.

While we have described the method of practising our invention and discussed particular embodiments, variations within the spirit and scope of the following claims may readily occur to persons skilled in the art:

We claim:

1. The method of stabilizing an arc column in an electric arc torch with a plasma forming gas comprising; in-

trodueing said gas at successive points along said column; and, at alternate points along said column, drawing off a portion of the gas so introduced such that said portion does not issue from the nozzle of said torch along with the working eliluent thereof.

2. An improvement for electric arc torches comprising a nozzle element providing an arc passageway, means in the form of apertures for introducing are stabilizing gases at successive points along said passageway, and secand means disposed at alternate points along said passageway for drawing off at least a portion of said gases from said passageway and removing same from said torch remote from the main passageway opening of said nozzle element.

References Cited by the Examiner UNITED STATES PATENTS 3,148,263 9/1964 Jensen 219-75 RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner. 

1. THE METHOD OF STABILIZING AN ARC COLUMN IN AN ELECTRIC ARC TORCH WITH A PLASMA FORMING GAS COMPRISING; INTRODUCING SAID GAS AT SUCCESSIVE POINTS ALONG SAID COLUMN; AND, AT ALTERNATE POINTS ALONG SAID COLUMN, DRAWING OFF A 